1. Field of the Invention
This invention relates to a synchronized light emitting device which is used, for example, for photographing.
2. Description of the Prior Art
Among various light emitting devices wherein energy for emission of light is accumulated in a main capacitor and then the accumulated charge is discharged through a discharge tube in order to cause the discharge tube to emit light, there is a type of light emitting device which includes a plurality of juxtaposed discharge tubes and allows such a selective operation that one of the discharge tubes is selectively caused to emit light or some or all of the discharge tubes are caused to emit light at a same time.
Now, construction in principle of a light emitting device of the type mentioned will be described with reference, for convenience of description, to a circuit diagram of FIG. 2 which shows a preferred embodiment of the present invention The circuit shown includes a main capacitor Co for accumulating energy for emission of light therein. The main capacitor Co is charged up to a predetermined high voltage by a charging circuit not shown. The circuit further includes a pair of xenon lamps Xe1, Xe2 for emitting light therefrom, a pair of triggering capacitors C1, C2 connected to be charged up to the same voltage with the main capacitor Co, and a pair of triggering transformers T1, T2. Now, if it is intended to cause only the xenon lamp Xe1 to emit light, a signal of a high voltage level is applied to a terminal T of the circuit with the charging of the capacitor C2 stopped in advance. Consequently, a silicon controlled rectifier SCR1 is turned on to cause the capacitor C1 to discharge so that an impulse of a high voltage is produced on the secondary side of the transformer T1 and triggers the xenon lamp Xe1 to discharge. To the contrary, in case it is intended to cause both of the xenon lamps Xe1, Xe2 to emit light at a same time, a signal of a high voltage level is applied to the terminal T with the capacitors C1, C2 charged up to the predetermined voltage in advance. Consequently, the silicon controlled rectifier SCR1 is turned on so that the xenon lamps Xe1, Xe2 are both triggered.
However, in such a conventional light emitting device, trigger electrodes of a plurality of discharge tubes are provided at like locations of the individual discharge tubes. Accordingly, when it is intended to cause one of a pair of adjacent discharge tubes to emit light and cause the other to remain not emitting light, if a trigger signal is applied to the one discharge tube so as to cause the same to emit light, sometimes the other adjacent discharge tube to remain not emitting light may also be triggered by induction.
As described above, when a xenon tube in a light emitting device is to be caused to emit light, a high voltage impulse is applied to a trigger electrode located outside the xenon tube so as to cause discharging in the xenon tube, whereafter the gas discharging within the xenon tube, is continued with discharge current flow from a main capacitor. However, when it is intended to cause a selected one of a plurality of xenon tubes to emit light, there is a problem that, if a high voltage impulse is applied to the trigger electrode of the selected xenon tube, a strong electric field is produced also around a trigger electrode of an adjacent xenon tube by electrostatic induction or by electric breakdown of air and triggers the adjacent xenon tube to emit light. A trigger electrode in a light emitting device is located adjacent a wall of a discharge tube as shown in FIGS. 3a or 3b. In FIGS. 3a and 3b reference symbol Q denotes a discharge tube, and g a trigger electrode, and FIG. 3a shows discharge tubes which each has a conductor ring as a trigger electrode wound around a portion near an end thereof at which a negative electrode is located while FIG. 3b shows another discharge tubes which each has a trigger electrode located along a side thereof and extending along the length thereof. In the arrangement shown in FIG. 3a, if a high voltage is applied only to the trigger electrode g1 of a left-hand side discharge tube Q1 in order to cause the discharge tube Q1 to emit light, a strong electric field is formed between the trigger electrode g1 and the cathode K1 of the discharge tube Q1 and causes discharging between the cathode K1 and the trigger electrode g1, thereby triggering discharging of the entire discharge tube Q1. In this instance, electric fields are produced as indicated in broken lines in FIG. 3a by electrostatic induction, and one of the electric fields which is produced between the trigger electrode g2 and the cathode K2 of the adjacent discharge tube Q2 may sometimes cause triggering of the adjacent discharge tube Q2. This also applies to the arrangement of FIG. 3b.